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The Pterobranchia of the Scottish National Antarctic Expedition (1902 to 1904)

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The Pterobranchia of the Scottish National Antarctic Expedition (1902 to 1904) ( 531 ) VII.—The Pterobranchia of the Scottish National Antarctic Expedition (1902 to 1904). By S. F. Harcner, Sc.D., F.R.S., Keeper of the Department of Zoology in the British Museum; and W. G. Ridewood, D.Sc, Lecturer on Biology in the Medical School of St Mary's Hospital, University of London. Communicated by Dr J. H. ASHWORTH. (With Five Text-Figures and Two Plates.)* (MS. received February 15, 1913. Read March 17, 1913. Issued separately July 4, 1913.) INTRODUCTION. The genus Cephalodiscus was instituted by M'INTOSH for a species, C. dodecalophus, which had been obtained by the Challenger Expedition. After the publication of the full account of this species, by M'INTOSH and HARMER (87) in the Challenger Report, succeeding papers for nearly twenty years were all based on the original Challenger material. The subject was in particular re-investigated by MASTERMAN in a series of papers (971, 972, 98, 99, 03). The Siboga report, published by HAKMER (05) in 1905, added three Oriental species to the genus. SCHEPOTIEFF (05, 07, 08) devoted several papers to a further description of the Challenger material; while, more recently (09), he has described an interesting new species, C. indicus, from Ceylon. In 1906 RIDEWOOD (06) described C. gilchristi from South Africa; and in the following year he gave an account (071) of the two species which had been dredged by the Discovery Expedition; and (07z) of the development of the plumes in four species of Cephalodiscus. One of the Discovery species, C. nigrescens, had been described two years earlier by LANKESTER (05), in a preliminary paper; and RIDEWOOD (L2) has recently brought forward evidence to show that this species had been dredged by the Erebus and Terror Expedition in 1841 or 1842. In 1907 ANDERSSON (07) added no less than six species to the genus, in describing the results of the Swedish South Polar Expedition, though we give some evidence (pp. 559-563) to show that one of them, C. insequatus, is synonymous with C. hodgsoni, which had been described 1 by RIDEWOOD (07 ) earlier in the same year, from the Discovery collection. The most recent addition to the list of species has been given by GRAVIER (12), who has published an account of C. anderssoni, a new species which was obtained by the second French Antarctic Expedition. Full references to the literature of the subject, up to the dates of the respective 1 publications, have been given in the works of HARMER (05), RIDEWOOD (07 ), and ANDERSSON (07). A general account of the group has more recently been given by SPENGEL (12). The characters of the ccenoecium are so well marked that it appears justifiable to * Published by permission of the Trustees of the British Museum. TRANS. ROY. SOC. EDIN., VOL. XLIX. PART III. (NO. 7). 70 532 DE S. F. HARMER AND DR W. G. RIDEWOOD ON THE use them, as a rule, as a ready means of distinguishing one species of Cephalodiscus from another. It is remarkable that what one would suppose to be the uncorrelated efforts of the numerous zooids of a colony do in fact produce so uniform a result. The comparison of larger series of specimens may perhaps show in the future that the coencecial characters are less reliable than they appear to be at present; but the fact remains that the common house of the colony has, as a rule, so distinct a character of its own that it is difficult to believe that it cannot be used for systematic purposes. It has thus been possible to distinguish the following subgenera of Cephalodiscus :— (1) Demiotheda, Ridewood (071), in which the coencecial cavity is continuous and the zooids occur separately or in groups in any part of it, being free to wander about in it. (2) Idiothecia, Ridewood (071), in which each zooid, with a certain number of its buds, occupies an independent tube-like cavity in the coenoecium. (3) Orthoecus, Andersson (07), in which each zooid has a tube of its own, but the tubes are free for the greater part of their length, instead of being embedded in the common ccenoecial mass as in Idiothecia. The descriptions- which have been published by the observers who have been referred to above show that there is a singular uniformity of structure in the zooid throughout the genus. In such fundamental characters as the three divisions of the body, and their associated coelomic cavities, the notochord, the proboscis-canals, the collar-canals, and the gill-slits, there is practically no variation. The remarkable character of the male C. sibogse, as described by HARMER (05), indicates that in that species at least there is a striking sexual dimorphism. In those other species in which both sexes are known there appears to be no essential difference between the two sexes except as regards their gonads. An exception must, however, be made in the case of C. ineequatus (probably = C. hodgsoni), where most of the female zooids have five pairs of arms, while most of the males have six pairs (see pp. 560-562). Making use of the ccenoecial characters which have been mentioned above, it appears to be possible to distribute the known species of Cephalodiscus among the three subgenera there indicated, and in this way to separate species which it might be hard to characterise from the structure of their zooids. But within a single sub- genus it becomes necessary to rely more largely on the characters of the zooid; and the discrimination of species on anatomical grounds may offer considerable difficulty. In the earlier accounts of species of Cephalodiscus some stress was laid on the proportions of the zooid and of its stalk. This procedure has been criticised by ANDEUSSON (07), who has had the unique opportunity of examining certain species in the living state. When account is taken of the highly muscular character of the zooid, and of the evidences of contraction afforded by wrinkling of the skin of the stalk and other parts, it is easy to believe that ANDERSSON'S criticism is to a large extent well founded. It may, nevertheless, be true that some weight may be ascribed in certain cases, as in discriminating between zooids in a similar degree of contraction, to features of this kind. PTEROBRANCHIA OF THE SCOTTISH NATIONAL ANTARCTIC EXPEDITION. 533 The difficulty of finding reliable specific characters in the structure of the zooid is increased by the fact that the material has as a rule not been preserved with any special refinements of technique. The study would be greatly facilitated by being able to make use of specimens which had been preserved in a fully extended condition. It would then be comparatively easy to ascertain the number of the arms or plumes with certainty. Under other circumstances this point, for instance, cannot always be made out without the possibility of mistake. Even in a well-prepared series of sections it is sometimes excessively difficult to count the arms, which may be cut in planes most unfavourable for study. The method of dissection is capable of giving valuable evidence; but here, too, mistakes are not out of the question. There is reason to believe that in some species of the genus the number of arms is variable 1 (of. RIDEWOOD, 07 ). It must also be remembered that in some cases the number of the arms has been given as the result of the study of sections of a very small number of individuals. The comparison of series of sections of the same species might suggest differences which are really due to the degree of maturity or of contraction, or to the planes in which the sections are cut. Some caution is thus necessary both in making statements and in accepting those which have been made by other observers. It can hardly be doubted, however, that the number of pairs of arms does provide a character which is of value systematically. Other characters which can be specially relied on are probably :—the number of buds, whether few or many, which are borne at any one time by the budding region of the stalk; the presence of one or three nerve-tracts in the stalk, as pointed out by HARMER (05) and ANDERSSON (07); the presence or absence of deep pigmentation of the skin ; the shape of the operculum or postoral lamella; the size of the free ova and the structure of the embryos; and the mode of development of the arms in the buds. Using some of these characters, the single species dredged by the Scotia may be thus characterised :— Cephalodiscus agglutinans, n. sp. Colony massive, branching, somewhat resembling that of G. nigrescens in the size of its branches, but the material of the ccenoecium includes large quantities of shells of Foraminifera, small fragments of shells of Mollusca and spines of Echinoids, and rounded particles of slate. The pieces of colony vary in size up to 100 or 115 mm. in greatest length, and 45 or 55 mm. in greatest breadth. Cavity of the coenoecium in the form of a tubular labyrinth continuous throughout (except for a few isolated septa), much branched in the interior, but the superficial parts of the tubes are radially set, and open at a fairly definite angle (about 80°) to the surface. Diameter of the peripheral tubes 1 mm. Ostia elliptical, size 1'2 by •8 mm., each with a single thick lip or spine* projecting about 3 mm. beyond the general surface. No peristomial tubes. Mean distance from the middle of an ostium to the middle of the ostium nearest to it, 3 or 4 mm.
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